Direct-drive implosion experiments on the GEKKO XII laser (9 kJ, 0.5 /xm, 2 ns) with deuterium and tritium (DT) exchanged plastic hollow shell targets demonstrated fuel areal densities (pR) of -0.1 g/cm 2 and fuel densities of -600 times liquid density at fuel temperatures of -0.3 keV. (The density and pR values refer only to DT and do not include carbons in the plastic targets.) These values are to be compared with thermonuclear ignition conditions, i.e., fuel densities of 500-1000 times liquid density, fuel areal densities greater than 0.3 g/cm 2 , and fuel temperatures greater than 5 keV. The irradiation nonuniformity in these experiments was significantly reduced to a level of <5°/o in root mean square by introducing random-phase plates. The target irregularity was controlled to a 1% level. The fuel pR was directly measured with the neutron activation of Si, which was originally compounded in the plastic targets. The fuel densities were estimated from the pR values using the mass conservation relation, where the ablated mass was separately measured using the time-dependent X-ray emission from multilayer targets. Although the observed densities were in agreement with one-dimensional calculation results with convergence ratios of 25-30, the observed neutron yields were significantly lower than those of the calculations. This suggests the implosion uniformity is not sufficient to create a hot spark in which most neutrons should be generated.
Hydrodynamic instabilities, such as the Rayleigh–Taylor (R–T) instability, play a critical role in inertial confinement fusion as they finally cause fuel-pusher mixing that potentially quenches thermonuclear ignition. Good understanding of the instabilities is necessary to limit the mixing within a tolerable level. A series of experiments has been conducted on the GEKKO XII laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 μm laser light. It has been found that (1) the imprint is reasonably explained by an imprint model based on the equation of motion with the pressure perturbation smoothed by the cloudy-day effect, and (2) the experimental R–T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.
A magnetic field B is found to be convected by the electron heat flux cf e because of the Nernst effect, i.e., with a velocity V T ( ~ 2^e/3n e T e ). In laser-driven ablation the convection is towards the overdense region and B can be amplified by 10-100 times because V • V T < 0, making B/n t and hence o) e r e almost constant for X mfp > c/w^. The spatially extended and amplified magnetic field can inhibit hot-electron transport, reducing preheat. For o) e T e > 1, the critical and ablation surfaces are brought closer together, which increases hydrodynamic coupling. PACS numbers: 52.25.Fi, 44.90. + c, 47.65.Fi In laser-driven fusion, uniform implosion is one of the key issues for successful performance. Among the sources of nonuniformity, an irradiation asymmetry has been believed to be smoothed out by electron thermal conduction. 1 But the inclusion of a self-generated magnetic field may change the situation. Several sources of such a magnetic field have been reported by many authors. 2 "" 9 In Refs. 2 and 3, the magnetic field is generated near the ablation region and in the others generated around the critical density.If we take no account of the effects of the thermal force, these magnetic fields are convected towards the underdense region by fluid flow. The most important point in considering the effect of a magnetic field is not where it is generated, but where it exists. In this context, the magnetic field transport becomes a critical issue. Some properties of the Nernst term have been suggested by earlier authors. Dolginov and Urpin 10 mentioned in their paper that the magnetic field will be convected to lower-temperature regions. Parfenov and Shishko 11 have shown that the transverse magnetic field profile in a magnetohydrodynamic shock is strongly affected by inclusion of the Nernst effect.In this Letter, we discuss the effect of the thermal force on the magnetic field transport and amplification, and we show that a magnetic field is transported to the overdense region by the electron heat flux. Furthermore we show that the magnetic field grows in magnitude to a value that is decided by the balance between convection and diffusion.To discuss these effects, we use the magnetohydrodynamic equations, which consist of conservation of mass, momentum, and energy, with the effect of the magnetic field as given by Braginskii. 12 The equation for the magnetic field is dt e n e 4ire Vx (VxB)xB -Vx IV7 , "T" IVW(1)where B, w, n e , P e , R T , and R u are the magnetic field, the ion velocity, the electron number density, the electron pressure, the thermal force, and the frictional force, respectively. The coupled partial differential equations described above are solved by our newly developed fluid particle-in-cell code 13 in one-or twodimensional plane geometry. Before presenting the detailed simulation results, let us analyze the thermal force term in the equation for the magnetic field. The fourth term in Eq. (1) can be rewritten as -Vx ft uT •vr, + v -B P'{X l + N m 0 VT P + V ca i ATrn}e 2 V5 + Vx c 2...
We propose a propulsion concept to drive a microairplane by laser that can be used for observation of climate and volcanic eruption. Since it does not have to develop thrust for vertical takeoff, and it has no engine in the normal sense, the microairplane can be very light, with its payload consisting only of observation and communication equipment. In order to demonstrate the concept, we succeeded in flying a paper microairplane driven by a 590 mJ, 5 ns pulse yttrium–aluminum–garnet laser that impinges on a double-layer “exotic target.” The coupling efficiency agrees well with simulations and with experiments.
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